Valve actuators find wide application, for example in the thermal and hydraulic power generation industries, in oil and gas extraction, marine, water utility and chemical processing industries. For the majority of these purposes the actuators are necessarily powerful and adapted to provide output levels of from about 3Nm to of the order of 1200Nm for rotary outputs or 100N to 30,000N for linear outputs. The valve actuators may be pneumatic, hydraulic, electric, and electro-hydraulic driven but are mainly pneumatic driven since these are less expensive to build to the required level of operational accuracy and reliability. The valve actuators generally all have processor control for setting, monitoring and controlling the actuator. Key control factors include actuator position, i.e. valve position, and actuator torque (primarily for rotary output actuators) and thrust (primarily for linear output actuators). In a rotary output actuator, for example, valve position is typically determined by counting the revolutions or part of a revolution of the driven rotary output shaft by a rotary encoder. The load generated at the actuator output shaft in such an actuator is typically determined by mechanical means, such as by a strain gauge or by a pressure transducer. Load might also be determined from a torque related current in the motor. Examples of latter such mechanisms are described in U.S. Pat. No. 4,288,665 and GB 2,101,355.
In addition to the above features, the valve actuators generally also have facility for failsafe if, for example, there is an electrical power failure. To this end, the majority of actuators further incorporate a compression spring return mechanism to restore the actuator to the desired failsafe position. This does, however, require use of a relatively high amount of power just to overcome the force of the return spring for normal operation.
An alternative approach is the use of stored electrical energy to supply enough power to drive a valve to a failsafe position under power failure conditions. Examples of this approach are disclosed in U.S. Pat. Nos. 5,278,454, 5,744,923 and GB-A-2192504.
A number of factors undermine efficient and effective operation of the valve actuator in normal use and these include gear wear, controller saturation, wind up and overshoot. The latter factors, that may be referred to as “stick-slip” factors, are commonly encountered with valve actuators and can prove a major impediment to maintaining efficiency in the closed loop processes implemented by control valve actuators, where rapid and often very small adjustments are required to be made to the valve position in order to maintain process control. The controller must firstly generate significant power to produce the required break away thrust or torque and then must rapidly attempt to control the valve into position. To overcome an initial heavy load, the controller will often be driven into saturation and, therefore, wind-up action (where the controller gives full power but the load instantaneously disappears and the controller is still fully powered) which will give rise to the valve overshooting its desired position and causing control loop instability.
Pneumatic driven valve actuators are, in common with the other valve actuators, mostly spring return for failsafe on loss of power and consequently are force balanced systems where the air pressure is matched to balance the spring force, valve stem friction, and valve stem force. This can lead to stick slip problems as when the air pressure is raised or lowered to move the valve by a small amount a sudden change in friction can cause the actuator to jump by more than the desired movement. They are also subject to unwanted movements due to changes in valve stem force as this unbalances the forces and the actuator needs to alter the air pressure to compensate.
Electric motor driven valve actuators are stiffer systems than pneumatics and don't suffer as much from changes to stem forces moving them away from their position. Electric motor driven valve actuators also suffer from stick slip problems although less so than pneumatics. However they can suffer from poor control due to backlash in the drive train. The motor needs to take up the backlash when reversing direction before the output starts to move. With a conventional system with one sensor at the output the controller will increase the motor speed, as it sees no movement at the output, and then when the backlash is taken up the output will suddenly overshoot especially if all that was required was a small movement. On current electric actuators the main approach to overcome this is to keep the backlash to a minimum by using high grade gears and a low gear ratio, often with an over-powered, high cost stepper (synchronous) motor. This and the long life requirements due to continuous operation make the drive train very expensive.
It is amongst objectives of the present invention to provide a control valve actuator that differs from and, suitably, improves upon the known such actuators and mitigates one or more of the aforementioned or hereinafter discussed operational difficulties.